Fluoroscopic screen which is optically homogeneous

ABSTRACT

A high efficiency fluoroscopic screen for X-ray examination consists of an optically homogeneous crystal plate of fluorescent material such as activated cesium iodide, supported on a transparent protective plate, with the edges of the assembly beveled and optically coupled to a light absorbing compound. The product is dressed to the desired thickness and provided with an X-ray-transparent light-opaque cover.

1 United States Patent Carlson 1 Nov. 4, 1975 FLUOROSCOPIC SCREEN WHICH IS OPTICALLY HOMOGENEOUS Roland W. Carlson, Lyndhurst, Ohio United States Steel Corporation, Pittsburgh, Pa.

Filed: Apr. 8, 1974 Appl. No.: 458,555

Inventor:

Assignee:

US. Cl. 250/483; 250/213 VT; 250/368 Int. Cl. G01T l/00 Field of Search 250/361, 362, 363, 366,

References Cited UNITED STATES PATENTS 7/1973 Martone 250/369 Primary ExaminerDavis L, Willis Attorney, Agent, or Firml-Iaro1d S. Meyer [57] ABSTRACT A high efficiency fluoroscopic screen for X-ray examination consists of an optically homogeneous crystal plate of fluorescent material such as activated cesium iodide, supported on a transparent protective plate, with the edges of the assembly beveled and optically coupled to a light absorbing compound. The product 7 is dressed to the desired thickness and provided with an X'ray'transparent light-opaque cover.

17 Claims, 5 Drawing Figures FLUOROSCOPIC SCREEN WHICH IS OPTICALLY HOMOGENEOUS BACKGROUND X-ray fluoroscopic systems are normally used for inspection of the internal structure of light-opaque products such as pneumatic tires, encapsulated electrical equipment, or welded metal products, using conventional fluoroscopic screens containing fine particles of calcium tungstate or other fluorescent material embedded in a transparent matrix or binder of organic plastic. Increasing the size of the fluorescent particles somewhat increases intensity of the image, but simultaneously decreases the attainable resolution of details.

It is also known that single crystal plates of fluorescent material can be used for visualizing X-ray shadow images, either directly or with the aid of an image intensifier (Pruitt Nucleonics Vol. 13, No. 8, pp. 2629, August 1955 and Carlson U.S. Pat. No. 3,356,851). However, such single crystal plates seem never to have gone into practical use except for scintillation cameras, in which very faint radiation from a radioactive source, such as a human organ which has selectively absorbed a radioactive isotope, is detected by a complex imaging system, as in the Martone U.S. No. 3,745,359.

SUMMARY OF THE INVENTION I have discovered that fluoroscopic X-ray screens with fine, sharp detail and excellent contrast can be made, using optically homogeneous crystal plates as the fluorescing material in a particular structure and arrangement.

The structure and arrangement of the fluorescent crystal plate according to this invention involves several special features in addition to the conventional X- ray-transparent but light-opaque backing on the blank face (that is, the face closest to the X-ray source) and light-transparent supporting and protective window on the opposite active face.

One such feature is optical coupling of the crystal plate and transparent window over their entire width, clear to the edge, and beveling of the edge of the assembly. Another feature is a light-absorbent material of high refractive index surrounding and in contact with the beveled edge.

The purpose of these two features, individually and together, is to eliminate the laterally directed, internally reflected light, which would otherwise continue to traverse the transparent assembly by internal reflection, until it is lost by optical absorption in the transparent materials or by scattering from optical defects in them or in their surfaces. The latter process is the major effect in reducing image contrast since this type of scattering can redirect this laterally directed light into the final image plane.

Neither feature alone will completely eliminate such stray light, which arises from the fact that fluorescence induced by X-rays is radiated in every direction. Only the rather narrow cone of light extending from a particular fluorescing point to the aperture of the camera lens system is useful. All other light, directed sidewards at every possible angle, is useless and must be elimias high an index of refraction as the value of 1.787 of cesium iodide reaches the parallel surfaces of the plate at angles of incidence exceeding the critical angle of about 34. This major portion will be transmitted, by one or more total reflections, to the edge of the assembly of crystal plate and support.

The purpose of the beveled edge is to increase the absorption of the light totally internally reflected to the edges of the plate over absorption of light reaching a square edge. A thorough ray trace analysis shows that the beveled edged directs this lateral light component to the light absorbent material with about twice the efficiency of a square edge. If the lateral light is not trapped at the edge, it will return by reflection into the crystal volume where it can encounter inevitable surface or volumeteric defects wherefrom the light can scatter into the image plane. Such scattered light will reduce the image contrast and thus decrease the sensitivity of the system.

To entrap and eliminate almost all of the internally reflected light, it is necessary to coat the beveled edge and the margin of the plane face opposing the beveled edge with a light-absorbing material. The refractive index of this material should be as high as possible. The amount of light energy passing into the absorbing material will depend on the refractive index of the material relative to that of the plate. The closer the index match, the greater the energy transfer and, thus, the greater the chance for absorption of the light by lightabsorbing pigment, such as carbon black, in the edge coating material.

The beveled edge does not have to have any particular kind of finish, since very beneficial results are obtained with either polished or rough surfaces, but best results have been obtained with rough surfaced beveled edges. This eliminates the need for a special operation of polishing the edges.

Another special feature of this invention involves the internal structure of the plate of fluorescent material. Such plates have previously been prepared by crystallizing a salt containing a small amount of an activating element, such as any alkali metal halide (sodium, potassium or cesium iodide) activated by a minor proportion of a different element (thallium, for example). During crystallization, the activating material as well as deactivating impurities can segregate and appear in higher proportions in some locations than in others. The consequence can be non-uniform intensity of fluorescence, which obscures the desired image. Moreover, if the impurities are of a kind causing afterglow or phosphorescence, non-uniform distribution causes still further spurious images.

According to this invention, fluorescent crystal plates can be prepared not only by direct crystallization but also by grinding and thoroughly mixing the fluorescent salt containing the desired activator followed by hot pressing, or by heating to a temperature of plasticity followed by plastic kneading and final pressing. The pressed slab is then finished to the desired size and shape and surface condition in the usual manner. Such crystal plates formed by mixing followed by hot pressing have very uniform properties and are essentially free from spurious images.

In addition to the foregoing, the mounting for the crystal plate assembly will normally include conventional internal flanges or glare stops surfaced with black non-reflecting coatings, to absorb the light which emerges from the window in a direction other than that of the optical system.

THE DRAWINGS In the accompanying drawings,

FIG. 1 is a diagrammatic view on a small scale of a fluorescent screen arranged for X-ray inspection of a large object.

FIG. 2 is a diagrammatic section on a larger scale of a fluorescent screen, and FIG. 3 is a similar section of one edge portion only on a much larger scale.

FIG. 4 is a partial diagrammatic section of a fluorescent screen arranged for X-ray inspection of a small object.

FIG. Sis a diagrammatic section of a modification.

DETAILED DESCRIPTION preferred to use one of the combinations known to fluorescebrightly in X-rays, such as cesium iodide activated by thallium, for example, a mixture of 10,000 parts cesium iodide with 13 parts thallium iodide (about one-tenth mol percent). This can be formed into a reasonably homogeneous crystal mass by crystallization from a melt in a cylindrical mold. A slab can be cut from the mass by a wet string, polished on one side, cemented to a glass plate by a clear adhesive, such as an epoxy cement, and then be ground and polished to the desired thickness.

Since the entire thickness of such a crystal plate could fluoresce in an X-ray beam, good resolution of a shadow picture requires a reasonable relation between thickness and diameter of the crystal screen, so that a fluorescing column of light produced by a narrow X-ray beam near an edge of the screen will not subtend an appreciable angle at the optical lens or other lightgathering or light-sensing element. The brightness increases with thickness of the plate, but the ability to resolve details of structure diminishes, so that a different relationship may be desirable for differing fineness of structure to be viewed. Diameters from to 100 times thickness are suitable, with about 25 times being a good compromise for examining structures of moderately fine detail with sufficient brightness for rapid and accurate discrimination.

Referring to FIG. 11, an X-ray source 10 is shown as being directed toward an object to be inspected, which may be a large welded metal structure 11 or the like, back of which is a light-tight imaging system including a fluorescent screen 12, a mirror 13 to reflect the image on the fluorescent screen to one side out of the X-ray beam to minimize damage to the optical and electronic elements by X-rays. Finally, a video camera 14 transmits the X-ray shadow picture to a more or less distant safe location for immediate viewing on a picture tube, or for recording for subsequent viewing.

0 tional anti-reflection quarter-wave coating 22. The

crystal plate 20 preferably has a diameter 20 to 50 times its thickness, or otherwise stated, is dressed and polished to a thickness 2 to 5% of its diameter. The edge of the fluorescent plate assembly including the glass support is finished to a bevel of about 45, preferably facing toward the X-ray source, and can be left in a rough ground finish without polishing.

The fluorescent plate assembly is suitably mounted in a ring 25 which may be flat metal like a washer or may be a thick ring cut away to leave a flange against which the assembly can be mounted. It is important that a light-absorbing coating 26 be in optical contact with the beveled edge, and also with the adjoining part of the plane surface which faces the beveled edge, to minimize edge reflection and to absorb as much as possible of the light which otherwise would be reflected back toward the center. For this purpose, the edge coating is applied both to the beveled edge and to the adjoining plane face inward from the edge to a diameter somewhat less than the smallest diameter of the beveled edge. The supporting ring 25 is accordingly made with an opening of about the same diameter as the clear center of glass plate 21 inside of the edge coating 26. Since the edge coating 26 covers the area facing the ring 25, it is convenient to make it of an adhesive material so that it will serve the dual function of absorbing light at the edge and of supporting the assembly of fluorescent plate 20 and glass plate 21 on the ring 25. Accordingly, it is preferred to make the edge coating 26 from an epoxy adhesive of high refractive index containing a light-absorbing pigment such as carbon black and to cement the glass plate 2 1 to the ring 25 by means of this adhesive. However, other supporting means such as clamps may be used, particularly if the screen is large and heavy and is likely to be subjected to vibration or shocks.

The fluorescent screen may be provided with a conventional mirror 27 such as aluminized polyester film, held against the crystal plate 20 by an X-ray transparent but light-opaque backing sheet. Such a mirror will increased image intensity, but at the expense of resolution of details, especially near the edges of thick crystal plates.

It is therefore often preferred to omit the mirror 27 and instead to continue the light-absorbent coating 26, in optical contact with crystal plate 20, clear across the back surface, as shown in FIG. 5. In this case, the backing 28 can be omitted.

As indicated in FIG. 3, light from any particular point 30 in the crystal plate 20 when it receives X-rays will fluoresce with visible light radiated in every direction. A light ray 31 perpendicular to the surface will emerge in the same direction. A light ray 32 at a small angle of incidence will be refracted, as indicated by the dash lines showing previous direction and solid lines showing refracted direction, both in passing from the crystal plate 20 into theglass plate 21 and in passing from the glass plate 21 into the atmosphere. Because of the presence of quarter-wave coating 22, very little of the light at angles less than the critical angle of incidence will be internally reflected to appear at an undesired location.

A light ray 33 at the critical angle of incidence will not emerge but will be totally internally reflected. A light ray 34 at considerably greater than the critical angle of incidence cannot emerge and will be totally reflected one or more times until it reaches the edge. There most of the light will be absorbed in edged coating 26, but some will be reflected. Because of the angle of the beveled edge, almost all reflected light will be directed toward the part of the opaque edge coating 26 which is on the plane face, and will pass through the quarter-wave coating 22 to be absorbed in the edge coating 26. 7

Thus essentially all of the useless light is absorbed, and only light radiating from the fluorescing points in the crystal plate at small angles of incidence can emerge. Consequently, the diffuse light which would limit the utility of crystal plate fluorescent screens, by fogging the shadow pictures, is eliminated by this invention.

This invention can be used in large sizes for examining large objects or in small sizes for examining small objects. The general arrangement will be the same in both cases, but some differences in details may be more convenient in one case or the other.

FIG. 4 shows an arrangement for using a crystal plate screen in X-ray inspection of small objects for which it may not be convenient to use a mirror to reflect the image to one side, out of the X-ray beam. In such cases, it will be necessary to use in the optical system types of glass which are not damaged by the X-ray frequencies used, and also video cameras and accessories which are not sensitive to X-rays, as is well understood.

The actual structure of the fluorescent screen is the same as in the description of FIG. 2 and FIG. 3, being only smaller in dimension, with a useful screen diameter in the range of one or two centimeters. It therefore contains the same crystal plate 20 cemented to a glass plate 21 with a quarter-wave coating 22 on its exposed surface, and a mirror 27 against the free surface of crystal plate 20, together with a light-opaque X-raytransparent backing sheet 28. It also has a beveled edge surrounded by a light-absorbing coating 26 and set in a supporting ring 25.

The ring together with the fluorescent screen assembly is suitably connected to a video camera 14 consisting of an optical lens 40 focused on the photocathode 41 of a video camera tube 43, such as an isocon or other conventional camera tube.

The light-tight housing connecting the fluorescent screen 12 to the video camera 14 preferably contains internal flanges 42 coated with light-absorbent material and functioning as light traps or glare shields to eliminate as far as possible the last traces of diffuse light which may have escaped from the edges of the fluorescent plate, along with all of the emerging light directed otherwise than into the aperture of the lens.

It is found that the crystal plate fluorescent screens of the kind described above produce visible images of X-ray shadow pictures of unparalleled brilliance,

For some special purposes, a combination of a reflective backing and an optically absorbent backing on the fluorescent crystal plate may be found useful. Thus a mirror may be located against the center of the fluorescent crystal plate where sharpest definition is obtained,

and replaced by a light absorbent coating on thej mar ginal portion. Conversely, the mirror may be placed against the marginal portion, with a central hole, either small or large, filled with such a coating for centering the imaging system or for other purposes. l

The fluorescent screens described above, with fluorescent crystal plates consisting mainly of alkali metal, halides, are best adapted for use with low voltage X- rays in examination of articles of low or medium den tion is preferred over electromagnetic radiation, for example in neutron fluoroscopy, a crystal material should be chosen which is suitably responsive to the particular radiation involved. Thus the lithium isotope of atomic mass 6 in the form of lithium iodide activated with europium is suitable for use with thermal neutrons and is commercially available.

In each of its forms this invention produces fluoroscopic images of unparalleled brillance and contrast, depicting the internal structure of objects with a clarity previously considered to be unattainable. This is particularly important for automatic inspection without visual observation by operators, in which a machine interpretation of the image to detect irregularities in density or position is required. The extremely good signal to noise ratio in electric signals produced from the fluorescent screens of this invention greatly facilitates mechanization of the X-ray inspection of manufactured articles, and the consequent elimination of operator errors as well as the labor cost of human operator inspection.

I claim:

1. A fluoroscopic screen comprising an assembly of a light-transparent supporting plate optically coupled by a light-transparent bonding layer to an optically homogeneous plate of light-transparent X-ray fluorescent material, the said assembly having a beveled edge.

2. A fluoroscopic screen as in claim 1, in which the thickness of the plate of fluorescent material is from about 2 to 5 percent of its diameter.

3. A fluoroscopic screen as in claim 1, in which the supporting plate is glass with a quarter-wave coating on its free face.

4. A fluoroscopic screen as in claim 3, in which the plate of fluorescent material is protected by an X-raytransparent cover.

5. A fluoroscopic screen as in claim 4, in which the beveled edge is surrounded by a coating of a coating of a light-absorbent material.

6. A fluoroscopic screen as in claim 5, in which the plate consists of an activated alkali metal halide.

7. A fluoroscopic screen as in claim 5, in which the coating extends completely across the free face of the fluorescent material.

8. A fluoroscopic screen as in claim 7, in which the plate consists of an activated alkali metal halide.

9. A fluoroscopic screen as in claim 1, in which the beveled edge is surrounded by a coating of a lightabsorbent material.

10. A fluoroscopic screen as in claim 9, in which the coating extends completely across the free face of the fluorescent material.

11. A fluoroscopic screen as in claim 9, in which a mirror faces the free face of the plate of fluorescent material.

12. A fluoroscopic screen as in claim 9, in which the free face of the fluorescent material is partly covered by a mirror and partly by a light-absorbent material.

13. A fluoroscopic screen as in claim 1, in which the plate consists of an activated alkali metal halide.

14. A fluoroscopic screen as in claim 1, in which a mirror faces the free face of the plate of fluorescent material.

15. An X-ray fluoroscopic system including a source of X-rays on one side of a space for an object to be examined, a fluoroscopic screen on the other side of the space, and an image detector coupled to the screen by an optical lens, characterized in that the fluorescent screen comprises an assembly of a light-transparent supporting plate optically coupled by a lighttransparent bonding layer to an optically homogeneous plate of light-transparent X-ray fluorescent material, and in that the said assembly has a beveled edge.

16. An X-ray fluoroscopic system as in claim 15 in which the supporting plate is glass with a quarter-wave coating on its free face, the plate of fluorescent material is protected by an X-ray-transparent cover, and the beveled edge is surrounded by a coating of a lightabsorbent material.

17. An X-ray fluoroscopic system as in claim 16 including additionally glare shields to absorb light emerging from the fluoroscopic screen in direction other than directly toward the lens aperture. 

1. A fluoroscopic screen comprising an assembly of a lighttransparent supporting plate optically coupled by a lighttransparent bonding layer to an optically homogeneous plate of light-transparent X-ray fluorescent material, the said assembly having a beveled edge.
 2. A fluoroscopic screen as in claim 1, in which the thickness of the plate of fluorescent material is from about 2 to 5 percent of its diameter.
 3. A fluoroscopic screen as in claim 1, in which the supporting plate is glass with a quarter-wave coating on its free face.
 4. A fluoroscopic screen as in claim 3, in which the plate of fluorescent material is protected by an X-ray-transparent cover.
 5. A fluoroscopic screen as in claim 4, in which the beveled edge is surrounded by a coating of a coating of a light-absorbent material.
 6. A fluoroscopic screen as in claim 5, in which the plate consists of an activated alkali metal halide.
 7. A fluoroscopic screen as in claim 5, in which the coating extends completely across the free face of the fluorescent material.
 8. A fluoroscopic screen as in claim 7, in which the plate consists of an activated alkali metal halide.
 9. A fluoroscopic screen as in claim 1, in which the beveled edge is surrounded by a coating of a light-absorbent material.
 10. A fluoroscopic screen as in claim 9, in which the coating extends completely across the free face of the fluorescent material.
 11. A fluoroscopic screen as in claim 9, in which a mirror faces the free face of the plate of fluorescent material.
 12. A fluoroscopic screen as in claim 9, in which the free face of the fluorescent material is partly covered by a mirror and partly by a light-absorbent material.
 13. A fluoroscopic screen as in claim 1, in which the plate consists of an activated alkali metal halide.
 14. A fluoroscopic screen as in claim 1, in which a mirror faces the free face of the plate of fluorescent material.
 15. An X-ray fluoroscopic system including a source of X-rays on one side of a space for an object to be examined, a fluoroscopic screen on the other side of the space, and an image detector coupled to the screen by an optical lens, characterized in that the fluorescent screen comprises an assembly of a light-transparent supporting plate optically coupled by a light-transparent bonding layer to an optically homogeneous plate of light-transparent X-ray fluorescent material, and in that the said assembly has a beveled edge.
 16. An X-ray fluoroscopic system as in claim 15 in which the supporting plate is glass with a quarter-wave coating on its free face, the plate of fluorescent material is protected by an X-ray-transparent cover, and the beveled edge is surrounded by a coating of a light-absorbent material.
 17. An X-ray fluoroscopic system as in claim 16 including additionally glare shields to absorb light emerging from the fluoroscopic screen in direction other than directly toward the lens aperture. 